Cover sheet cleaning means for a contact printer

ABSTRACT

An improvement in a contact printer which comprises means for supporting a film and an original in superposed relationship on a vacuum table, means for providing a source of light for exposing the film through the original, and means for moving the light over the surface of the superposed film and original. A substantially impervious, flexible and transparent cover sheet carried by the light moving means is arranged to cover the superposed film and original as the exposing light is scanned across the film and original whereby the vacuum is applied to the film and the original to remove air from therebetween as the film is exposed by the light through the cover sheet and the original. The improvement comprises means for contacting the surface of the cover sheet during use and removing dust and dirt thereon to assure that the optical path for the exposing light remains clear and unobstructed.

RELATED APPLICATIONS

The present application is related to our co-pending applicationsentitled A CONTACT PRINTER FOR EXPOSING SENSITIZED GRAPHIC ART FILM ANDPAPER, Ser. No. 438,565, A METHOD OF EXPOSING SENSITIZED GRAPHIC ARTFILM AND PAPER, Ser. No. 438,564, A COMPACT LIGHT COLLIMATOR FOR ASCANNING CONTACT PRINTER, Ser. No. 440,577, A COMPACT CONTACT PRINTERWITH A FLEXIBLE TRANSPARENT COVER SHEET, Ser. No. 438,822, and AREMOVABLE COVER SHEET ROLL FOR A CONTACT PRINTER, Ser. No. 438,567, allfiled on even date herewith.

BACKGROUND OF THE INVENTION

Contact printing or exposure is a traditional graphic arts procedure forgenerating same size, high resolution reproductions of line and halftonefilm images. This technique ensures faithful original film imagereproduction by placing the original film image in intimate contact witha receiving film or paper emulsion and, in the presence of a vacuum tohold them in intimate contact throughout the exposing process, exposingthrough the original film image to the receiving film or paper by meansof a point or reflected light source. Typically, these images are of arelatively large size in order to meet the needs of the graphic artsindustry and thus require large format originals and receiving films andthe equipment necessary to handle such sized films.

Traditional silver halide contact films have sufficient photographicvisible light sensitivity that the contacting exposure can be carriedout with a simple, low cost, low energy tungsten light source. However,because of the sensitivity of these films to visible light, the contactexposure must be carried out in a darkroom environment. This causes amajor inconvenience for the operator and results in a loss ofproductivity in the graphic arts production process.

In an attempt to improve graphic arts contact exposure productivity,film manufacturers have developed normal roomlight handling contactfilms. These films are photographically very insensitive to visiblelight, but have a relatively high UV sensitivity (although, in fact,having a relatively low UV sensitivity compared to the UV sensitivitiesof darkroom-handled film). These roomlight-handled films have improvedthe productivity of graphic arts contact exposure operations byeliminating the bottlenecks of the darkroom environment. However, theyhave also increased the use of high intensity, expensive, high UV outputlight sources. Metal halide light sources are a common type of UV lightsource used to expose these films.

One of the more important aspects of the graphic arts contact exposingor printing process is the need for accurately reproducing very finelines and halftone dots. This has resulted in the requirement that theexposing light be collimated at the exposing plane so that such linesand dots can be reproduced accurately without undercutting or spreadingand changing the size of the reproduced line or dot and the resultingimage. Adequate light collimation has been achieved in the past by usinga point light source mounted at a sufficiently great distance from theexposing plane that the light is substantially collimated by the time itreaches the exposure plane.

Given the large exposing plane area necessary for graphic arts purposes,the need for a high degree of light collimation to achieve accurateimage reproduction, and the relative low photographic UV sensitivitiesof roomlight handling contact films, a high energy, expensive metalhalide source has heretofore been required to expose these films. Thesehigh intensity UV light sources have required that the operator beshielded from the light source to avoid exposure to harmful UVradiation. Examples of such arrangements are illustrated in U.S. Pat.Nos. 4,029,404, 4,316,669, and 4,437,759. As a result, the total volumeof space required for the graphic arts contact exposing set-ups of theprior art has been large and, since most printing shops have more thanone of these exposing set-ups, the total volume of space required issignificant.

It has been found that one of the limiting factors to increasing theproductivity of graphic arts contact printers is that contact printersof the prior art have required that the exposure of the film wait untilthe film has been drawn into intimate contact throughout the entireextent of the interface with the original before the exposure cancommence. A contact printer which permits the exposure to begin beforethe entire film/original laminate has been evacuated has been developedwhich employs a film cover sheet which is deployed over theoriginal/film laminate and through which the film is exposed. It hasbeen found that with use this cover sheet can become scratched anddirty, interferring with the accurate reproduction of graphic artsimages.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an improvement in a contactprinter which comprises means for supporting a film and an original insuperposed relationship on a vacuum table, means for providing a sourceof light for exposing the film through the original, and means formoving the light over the surface of the superposed film and original. Asubstantially impervious, flexible and transparent cover sheet carriedby the light moving means is arranged to cover the superposed film andoriginal as the exposing light is scanned across the film and originalwhereby the vacuum is applied to the film and the original to remove airfrom therebetween as the film is exposed by the light through the coversheet and the original. The improvement comprises means for contactingthe surface of the cover sheet during use and removing dust and dirtthereon to assure that the optical path for the exposing light remainsclear and unobstructed.

Further, an improvement in a contact printer is provided which comprisesvacuum table means for supporting a photosensitive film and an originalcontaining an image to be reproduced in superposed relationship, lightmeans for providing an elongated beam of photoactive light for exposingthe film through the original, means for moving the light meanstransversely of the length of the beam over the vacuum table surfaceover the superposed film and original whereby the superposed film andoriginal are scanned by the light so that light exposes the film throughthe original. Means carried by said light moving means is provided forremovably covering the superposed film and original with a substantiallyimpervious flexible and transparent polymeric cover sheet as theexposing light is scanned across the film and original whereby thevacuum is applied to the film and the original to remove air fromtherebetween as the film is exposed by the light through the cover sheetand the original. The improvement comprises a pair of rollers contactingeach surface of the cover sheet during use and removing dust and dirtthereon to assure that the optical path for the exposing light remainsclear and unobstructed. The rollers are rotatable and driven by contactwith the cover sheet and being formed of a single component urethane.

Various means for practicing the invention and other features andadvantages thereof will be apparent from the following detaileddescription of an illustrative preferred embodiment of the invention,reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred embodiment of the presentinvention;

FIG. 2 is a cross-sectional view of the light trolley taken along line2--2 of FIG. 1;

FIG. 3 is a perspective view of a portion of the light collimator; and

FIG. 4 is a perspective view of the cover sheet applying assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, a preferred embodiment of a contactprinter 10 embodying the present invention is illustrated. The contactprinter comprises a platen member 12 having a planar upper surface 14with dimensions slightly larger than the maximum dimensions of thelargest original to be copied thereon. A light carriage 16, having alength approximately equal to one dimension of the platen member 12, isarranged to be moved across the platen transversely to the length of thecarriage, thereby providing coverage of the entire platen during itsmovement.

The upper surface of the platen member 14 forms a vacuum table for thefilm 18 and the original 20 which are superposed thereon for exposure ofthe film (see FIG. 2). A rigid chamber 22, having a thickness ofapproximately one inch, is formed beneath the vacuum table. A pluralityof vacuum ports 24 are formed in the film/original accepting portion ofthe surface 14 and communicate between the surface and the interior ofthe chamber. With the exception of the vacuum ports 24, the chamber 22is sealed so that a vacuum pump (not shown, but normally disposed remotefrom the apparatus) can pull a vacuum within the chamber which is thenapplied, via ports 24, to the film and original which are placed on thesurface 14. The ports 24 have a diameter of approximately 0.040 inchwhich is sufficiently large to effectively apply the vacuum to thefilm/original disposed on the surface 14, but is sufficiently small andthe capacity of the vacuum pump is great enough that it will maintainthe vacuum even though the size of the film/original is less than themaximum, leaving some of the vacuum ports uncovered. The vacuum portsoutside of the area covered by the maximum size film/original areconnected by shallow grooves 26 having a cross section of 0.015 inch by0.015 inch which permit air trapped between the film/original and acover sheet (to be further described hereinbelow) to be evacuated. Ithas been found that satisfactory operation can be achieved with a vacuumpump which is capable of flow rates of between 15 and 30 cfm whilegenerating a vacuum within the chamber of between 8 and 12 inches ofwater.

The light carriage 16 is disposed on the upper surface of the vacuumtable and is arranged to be moved across the table from a home position(the rear of the vacuum table in the example illustrated) to theopposite edge of the table, and returned to the home position. Thecarriage is provided with wheels (not shown) at each end thereof whichride upon tracks (not shown) provided at the lateral edges of the platensurface 14 or within the platen. The carriage is driven by a drive screw28 within and extending the length of the platen which engages anddrives a drive-nut 30 connected to the carriage. The drive screw isdriven by a variable speed, reversible motor (not shown) located withinthe platen.

The light carriage 16 carries one or more high UV output fluorescentaperture lamps 32, with an associated mechanical light collimator 34positioned beneath each lamp, a roll of a transparent cover sheet 36, anassociated pressure roller 38, and a pair of cleaner rolls 40 and 42.

The fluorescent lamps 32 are long tubular bulbs extending the length ofthe carriage. The phosphors which coat the inner walls of the bulbs areselected to generate light having a wavelength in the range of 350 nm to460 nm, which matches the sensitivity of the film being exposed. Thebulbs have an internal reflective layer between the phosphor and theenvelope glass which covers a major angular portion of the envelope walland reflects a high percentage of the radiation striking it. A clearwindow 44, having an aperture of about 60° of the bulb circumference, isprovided along the length of the bulb with no phosphor or reflectivelayer. This window produces an elongated, concentrated beam of highluminance UV light output along the length of the bulb because thereflective coating directs the major part of the bulb output through theuncoated window. The lamps are selectively oriented in the carriage withthe windows 44 directed downwardly so that the beam of concentratedlight is directed toward the sensitized film material 18.

The mechanical light collimator 34 associated with each bulb comprises aplurality of parallel, fine collimator tubes 46 which are closely spacedand which transmit parallel rays of light from the fluorescent aperturelamp to the exposure plane. Inasmuch as image quality is related to thesolid angle of the light incident on the original/film laminationsurface, based on a given distance between the sensitized film emulsionand the image of the original, the equivalent diameter of the tubes andtheir length are selected to extinguish incident angles of light at theexposure plane greater than a predetermined solid angle, for example asolid angle of between about 3° and about 9°. Moreover, since theillumination at the film surface depends upon the light output of thelamp and the distance of the lamp from the film surface, for a givenlamp output, the shorter the length of the collimator, the greater theexposure power at the film. A preferred embodiment employs hexagonaltubes having an equivalent diameter of 0.125 inch and a length of 0.5inch, with the bottom of the tube spaced above the film plane a distanceof 0.5 inch.

The transparent cover sheet 36 has dimensions substantially equal to thewidth and length of the vacuum table and is supported as a roll 48mounted in the carriage 16 parallel with and ahead of the fluorescenttubes (as viewed with respect to the initial travel of the carriage).The outer, free end of the cover sheet is releasably connected to thevacuum table as indicated at 50. The cover sheet is wrapped on a core 52which is removably held at each end by hub members, 54 and 56, rotatablymounted in the carrier. One hub 54 is connected to one end of a constantforce tension spring 58 which acts to apply tension to the roll of thecover sheet material as it is unwound and wound by movement of thecarriage across the vacuum table, as will be further describedhereinbelow. The other end of the tension spring is connected to andwrapped on a storage drum 60 where it remains in its idle position.Through the unwinding force generated as the cover sheet is unwound, thespring is transferred onto the core hub 54, and produces the windingtorque moment to the shaft of the core to rewind the cover sheet whenthe carriage movement is reversed. The other hub 56 is spring loadedalong the core axis by an axial compression spring 62. The axialcompression spring permits the hub 56 to be moved axially, disengagingthe cover sheet roll to permit its easy replacement. Thus, when thecover sheet becomes damaged, worn, scratched, dirty or translucent, andthus would adversely affect the exposure light directed therethrough, itcan be replaced with a new cover sheet.

The cover sheet 36 is formed of an impermeable sheet of a polymericmaterial which is substantially transparent to UV radiation. Thematerial also preferably resists abrasion and possesses anti-staticproperties, or is treated to obtain these properties, so that it resiststhe accumulation of dust, dirt, and other materials which wouldadversely affect the transmission of exposing light therethrough, orwhich would adversely affect the collimation of the light. It has beenfound that a sheet of clear polyester sheeting having a thickness ofapproximately 0.004 inch satisfactorily meets these requirements. Thesheeting may be provided with anti-static properties via coatings orother treatments to minimize the static attraction of dust and dirt tothe cover sheet, to prevent the cover sheet from "clinging" to thefilm/original laminate, and to prevent static discharges which canimpart an unwanted exposure to the film. The sheeting can also beprovided with an abrasion resistant coating to minimize scratching ofthe cover sheet which would adversely affect its "clear" opticalproperties. Still further, the cover sheet can be provided, on thesurface thereof which contacts the film/original laminate, with anoptically clear matte finish which facilitates the ready application ofthe vacuum to the cover sheet/original/film laminate by providingminiscule air passages for the removal of air. The cover sheet acts as a"vacuum lid" as it is unwound over the original/film laminate duringvacuum drawdown as the carriage is moved across the vacuum table toexpose the film. The cover sheet also protects the original/filmlaminate underneath from any abrasive action of the pressure roller 38as it forces the cover sheet/original/film lamination into intimatecontact to remove any air from the lamination which might otherwisedistort the resulting exposure.

Because of its importance to the satisfactory exposure of the contactprint, the cover sheet must be kept clean. As an aid to keeping thecover sheet optically clean and free from dust and lint, the cleanerrolls 40 and 42 are mounted transversely of the light carriage 16between the cover sheet roll 48 and the pressure roller 38 in engagementwith the front and rear surfaces of the cover sheet 36. The cleanerrolls are rotatable and are driven by contact with the cover sheet as itis unwound over the film/original laminate and rewound during eachexposure cycle. The cleaner rolls are formed of a single componenturethane having 20 durometer Shore A hardness which clean the coversheet without scratching.

OPERATION

Initially, the light carriage 16 is parked at the rear of the vacuumtable 14. A sheet of film or paper 18 is positioned emulsion side-up onthe platen. The longest sheet dimension is preferably perpendicular tocarriage motion to minimize the carriage travel necessary to fully coverthe sheet and thus keep the time required to a minimum. The vacuum pumpis activated and applies the vacuum to the film through the ports 24 tohold the film to the platen surface. The original 20, such as a colorseparation, is then placed on top of the film. The separation can bepositioned by means of registration pins (not shown). If registrationpins are used, they are located along the edge of the original/filmlaminate closest to the carriage home position. The type of film orpaper to be exposed, or the exposure speed thereof, is entered into themachine controller. The film information and lamp intensity readings arethen used to calculate the carriage traverse speed. When the "start"button is actuated, the carriage 16 moves forward toward the front edgeof the platen 14 at the proper dot-for-dot exposure speed. While thecarriage moves forward, the clear anti-static cover sheet 36 is unwoundto cover the original/film lamination. The pressure roller 38 pushes outany remaining air from the lamination and the cover sheet is drawn intointimate contact with the lamination by the vacuum through the ports 24.The carriage continues until the "end of travel" position is reached andreverses direction to finish exposing the film and return to the homeposition. As the carriage returns to the home position, the cover sheet36 is rewound on the core 52 by the constant tension spring 58,uncovering the original/film lamination after exposure for that area hasbeen completed. It is apparent that with small film sizes, completetraverse of the platen by the carriage is not necessary. The "end oftravel" position is selectable and can be specified by the user. Whenthe carriage is back at home position, the vacuum and the lamps are shutoff or placed on standby, and the film and separation can be removed.

The invention has been described with reference to specific embodimentsand variations, but it should be apparent that other modifications andvariations can be made within the spirit and scope of the invention,which is defined by the following claims.

We claim:
 1. In a contact printer which comprises means for supporting afilm and an original in superposed relationship on a vacuum table, meansfor providing a source of light for exposing said film through saidoriginal, means for moving said light over the surface of saidsuperposed film and original, a substantially impervious flexible andtransparent cover sheet carried by said light moving means arranged tocover said superposed film and original as said exposing light isscanned across said film and original whereby the vacuum is applied tosaid film and said original to remove air from therebetween as said filmis exposed by said light through said cover sheet and said original, theimprovement comprising means for contacting the surface of said coversheet during use and removing dust and dirt thereon to assure that theoptical path for the exposing light remains clear and unobstructed.
 2. Acontact printer according to claim 1 wherein said contacting meanscomprises a roller member.
 3. A contact printer according to claim 1wherein said contacting means comprises a pair of roller members each ofwhich contacts one surface of said cover sheet.
 4. A contact printeraccording to claim 2 wherein said roller member is formed of a polymericmaterial.
 5. A contact printer according to claim 4 wherein saidpolymeric material is a single component urethane.
 6. A contact printeraccording to claim 1 wherein said roller member is rotatable and isdriven by said cover sheet.
 7. In a contact printer comprising vacuumtable means for supporting a photosensitive film and an originalcontaining an image to be reproduced in superposed relationship, lightmeans for providing an elongated beam of photoactive light for exposingsaid film through said original, means for moving said light meanstransversely of the length of said beam over said vacuum table surfaceover said superposed film and original whereby said superposed film andoriginal are scanned by said light so that light exposes said filmthrough said original, means carried by said light moving means forremovably covering said superposed film and original with asubstantially impervious flexible and transparent polymeric cover sheetas said exposing light is scanned across said film and original wherebysaid vacuum is applied to said film and said original to remove air fromtherebetween as said film is exposed by said light through said coversheet and said original, the improvement comprising a pair of rollerscontacting each surface of said cover sheet during use and removing dustand dirt thereon to assure that the optical path for the exposing lightremains clear and unobstructed, said rollers being rotatable and drivenby contact with said cover sheet, said rollers being formed of a singlecomponent urethane.